CDMA based systems operate in a full-duplex mode. That is, they transmit and receive signals at the same time. The transmitter and receiver signal paths usually use a common antenna and are separated by a duplex filter, commonly called a duplexer. Unfortunately, duplexers do not provide perfect signal isolation between the transmitter and the receiver thereby allowing the receiver to experience transmitter signal leakage. This particular problem is exacerbated due to the fact that the size of CDMA transceivers, such as cell phones, and their components have been shrinking thereby tending to further lower the transmitter-receiver isolation attainable in the duplexer.
The receiver typically employs a low noise amplifier as a front end, which is connected through the duplexer to the common antenna. The low noise amplifier, to a large extent, establishes the noise figure of the receiver. The noise contribution of each block in the receiver chain is reduced by the total gain of the blocks preceding it. Therefore, for minimum receiver noise factor, the gain of the low noise amplifier is usually maximized and its noise factor is minimized based on other receiving block constraints. However, at maximum transmitter output power, the transmitter signal leakage can cause the low noise amplifier to saturate due to this large gain thereby imposing difficult constraints for the receiver design.
Accordingly, what is needed in the art is an enhanced way to accommodate transmitted energy interference in a receiver portion of a full duplex transceiver.